The present invention relates to the deposition of halogen-doped dielectric layers during wafer processing, and more specifically to a method and apparatus for forming a high deposition rate halogen-doped silicon oxide layer having a low dielectric constant and high film stability.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to as chemical vapor deposition or xe2x80x9cCVD.xe2x80x9d Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions produce a desired film. The high temperatures at which some thermal CVD processes operate can damage device structures having metal layers.
Another CVD method of depositing layers over metal layers at relatively low temperatures includes plasma enhanced CVD (PECVD) techniques. Plasma CVD techniques promote excitation and/or dissociation of the reactant gases by the application of radio frequency (RF) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the required temperature for such CVD processes. The relatively low temperature of a PECVD process makes such processes ideal for the formation of insulating layers over deposited metal layers and for the formation of other insulating layers.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two year/half-size rule (often called xe2x80x9cMoore""s Lawxe2x80x9d), which means that the number of devices that will fit on a chip doubles every two years. Today""s wafer fabrication plants are routinely producing integrated circuits having 0.5-xcexcm and even 0.35-xcexcm features, and tomorrow""s plants soon will be producing devices having even smaller geometries.
As devices become smaller and integration density increases, issues that were not previously considered important by the industry are becoming of concern. With the advent of multilevel metal technology in which three, four, or more layers of metal are formed on the semiconductors, one goal of semiconductor manufacturers is lowering the dielectric constant of insulating layers deposited between the metal layers. Such layers are often referred to as intermetal dielectric (IMD) layers. Low dielectric constant films are particularly desirable for IMD layers to reduce the RC time delay of the interconnect metallization, to prevent cross-talk between the different levels of metallization, and to reduce device power consumption.
Many approaches to obtain lower dielectric constants have been proposed. One of the more promising solutions is the incorporation of fluorine or other halogen elements, such as chlorine or bromine, into a silicon oxide layer. It is believed that fluorine, the preferred halogen dopant for silicon oxide films, lowers the dielectric constant of the silicon oxide film because fluorine is an electronegative atom that decreases the polarizability of the overall SiOF network. Fluorine-doped silicon oxide films are also referred to as fluorinated silicon glass (FSG) films.
FSG films may be deposited using fluorine sources such as CF4, C2F6, and NF3. One particular method of depositing an FSG film forms a plasma from a process gas that includes silicon tetrafluoride (SiF4) as the fluorine source, silane (SiH4) and O2 precursors. It is believed that SiF4 is a particularly effective fluorine source for FSG films because the four fluorine atoms bonded to a silicon atom in a molecule of the gas supply a higher percentage of fluorine into the deposition chamber for a given flow rate as compared with other fluorine sources. Additionally, SiF4 has more fluorine bonded to silicon available for the plasma reaction than other fluorine sources. The use of SiF4 as a source of fluorine for FSG films is discussed in more detail in U.S. Ser. No. 08/538,696, filed Oct. 2, 1995, entitled xe2x80x9cUSE OF SIF4 TO DEPOSIT F-DOPED FILMS OF GREATER STABILITYxe2x80x9d; and to U.S. Ser. No. 08/616,707, filed Mar. 15, 1996, entitled xe2x80x9cMETHOD AND APPARATUS FOR IMPROVING FILM STABILITY OF HALOGEN-DOPED SILICON OXIDE FILMSxe2x80x9d. The Ser. Nos. 08/538,696 and 08/616,707 applications are assigned to Applied Materials Inc, the assignee of the present invention.
Thus, manufacturers desire to include fluorine in various dielectric layers and particularly in intermetal dielectric layers. One problem encountered in the deposition of FSG layers is film stability. Loosely bound fluorine atoms in the lattice structure of some FSG films result in the films having a tendency to absorb moisture. The absorbed moisture increases the film""s dielectric constant and can cause further problems when the film is exposed to a thermal process such as an anneal process.
The high temperature of the thermal processes can move the absorbed water molecules and loosely bound fluorine atoms out of the oxide layer through metal or other subsequently deposited layers. The excursion of molecules and atoms in this manner is referred to as outgassing. Such outgassing can be determined by detecting fluorine, hydrofluoric acid (HF) or H2O leaving the film as the film is heated to a specified temperature. It is desirable to have little or no outgassing at temperatures up to at least the maximum temperature used during substrate processing after the FSG film has been deposited (e.g., up to 450xc2x0 C. in some instances).
Generally, the dielectric constant of an FSG film is related to the amount of fluorine incorporated into the film. An increase in the fluorine content of the film generally decreases the dielectric constant of the films. FSG films having a high fluorine content (e.g., above 7 or 8 atomic percent [at. %] fluorine), however, are more likely to have moisture absorption and outgassing problems than films of a lower fluorine content (e.g., lower than 7 or 8 at. % fluorine). Therefore, the development of oxide films having low dielectric constants are necessary to keep pace with some emerging technologies.
In addition, a method of increasing the stability of halogen-doped oxide films, and in particular, high fluorine content FSG films, thereby reducing moisture absorption and outgassing in the films, is also desirable.
Another concern of manufacturers is the throughput of the process. In order to have a high throughput, the deposition rate of the process has to be high. Hence, in addition to being stable, the film should have a high deposition rate to enhance deposition efficiency.
The present invention provides a halogen-doped layer having a low dielectric constant and improved stability even at high halogen-doped levels. The invention also provides a method and apparatus for forming such a layer at a high deposition rate. Film stability is improved by introducing a nitrogen source gas and a halogen source gas into a deposition chamber along with silicon and oxygen sources. A plasma is then formed from the gases to deposit a halogen-doped layer over a substrate disposed in the chamber. It is believed that the introduction of the nitrogen source reduces the amount of free or loosely bonded fluorine in the layer, thereby enhancing the stability of the layer.
An FSG film is deposited according to a preferred embodiment of the method of the present invention. In this embodiment, the nitrogen source gas is N2, and the halogen source gas is SiF4. The oxygen source may be from N2O, and the silicon source is SiH4. The ratio of N2 to SiF4 is between about 3 to 20, and the ratio of N2 to SiH4 is between about 3 to 10. In addition, the ratio of N2 to N2O is between about 0.5 to 4. An FSG film deposited according to this embodiment can incorporate up to at least 16 at. % fluorine as measured using Secondary Mass Ion Spectroscopy (SIMS). In addition, the film shows substantially no fluorine or HF outgassing from the layer when heated to a temperature up to at least 475xc2x0 C. and 500xc2x0 C., respectively. In a more preferred embodiment of the method of the present invention, near the completion of the deposition step, the flow of SiF4 is stopped several seconds before the flows of the other process gases are stopped. Employing this sequence helps further reduce loosely bonded fluorine in the film and allows for the deposition of an FSG film that has up to at least 16 at. % fluorine and shows substantially no fluorine, HF, or H2O outgassing from the layer when heated to a temperature up to at least 700xc2x0 C.
These and other embodiments of the present invention, along with many of its advantages and features, are described in more detail in the text below and the attached figures.